The invention concerns a device for alternating examination of a measurement object by means of MPI (magnetic particle imaging) and by means of MRI (magnetic resonance imaging), comprising at least two elements that generate a magnetic field, the device having a first volume under investigation for MRI operation, in which a homogeneous field is generated, and also a second volume under investigation for MPI operation, in which a spatially strongly varying magnetic field profile is generated, the field vectors of which are different with respect to direction and/or magnitude at all spatial points and that have a field magnitude of zero at one spatial point, wherein the device comprises a resistive drive field coil system for generating a drive field, and also a resistive MRI gradient coil system.
A device of this type is disclosed by reference [3] (Weizenecker et al., 2009).
A large number of tomographic imaging methods have been invented during the last decades, such as e.g. computer tomography (CT) by Hounsfield in 1969, magnetic resonance imaging (MRI) by Lauterbur and Mansfield in 1973 or positron emission tomography (PET) by Ter-Pogossian and Phelps in 1975. Imaging methods have become more and more important in today's medical diagnostics due to continuous further development of the hardware, the sequence and/or reconstruction algorithms. The diagnostic informative value using imaging methods could be further increased through combination of individual imaging methods into so-called hybrid systems (e.g. PET-CT in clinical operation since 2001 and MRI-PET in clinical operation since 2010). All hybrid systems are based on synergetic combination and/or graphic superimposition of complementary information of the individual modalities. Thus, CT data of a PET-CT hybrid system is used e.g. for morphological information and also for attenuation correction of the PET data.
Gleich and Weizenecker invented a further tomographic imaging method in 2001 called magnetic particle imaging (MPI) (DE10151778A1). This young and rapidly developing volumetric imaging method is used for detecting the spatial distribution of applied superparamagnetic nanoparticles (SPIO). This method provides spatial resolution as well as high temporal resolution (cf. references [1-3]).
The basic principle of MPI is based on excitation of the nanoparticles by means of a temporally varying magnetic field, the so-called “drive field” (DF), with an excitation frequency f0. Due to the non-linear magnetization curve of the SPIOs, the particle responses are harmonics of f0, which are detected by means of receiver coils and are utilized for image reconstruction. Since tissue provides a negligibly small and non-linear response to the excitation frequency f0, this method offers a large contrast due to acquisition merely of the particle response. Spatial encoding is based on the effect that particle magnetization is saturated starting from a specific magnetic field strength. Due to magnetic excitation with the frequency f0, the magnetization of the saturated SPIOs changes only minimally and these changes then do not or only hardly contribute to the particle response. In order to utilize this saturation effect, a static magnetic field gradient, the so-called “selection field (SF)”, is generated with a field-free point (FFP). Departing from this FFP, the magnetic field strength increases in all spatial directions.
A magnetic field behavior of this type can e.g. be generated by permanent magnets with opposite magnetization direction or by means of a Maxwell electromagnetic coil pair. Due to the saturation effect, only particles in the direct vicinity of the FFP are excited and thus contribute to the particle response. The extension of the FFP, and therefore the sensitivity of the MPI method depends on the magnetic field strength at which the particles reach saturation and also on the gradient strength of the SF with which the magnetic field rises starting from the FFP (cf. references [4, 5]). In order to allow volumetric imaging, the FFP is controlled in relation to the object under investigation by e.g. superposition of additional magnetic fields and/or by mechanical movement of the object under investigation.
The quantitative MPI method offers promising possibilities for non-invasive applications in the field of molecular and medical imaging such as e.g. cell tracking or cancer diagnosis and also in the field of cardiovascular diagnosis and blood vessel imaging due to its high sensitivity and its high temporal resolution. In contrast to other imaging methods such as e.g. CT and MRI, the acquired current MPI image data sets still have a relatively low spatial resolution in the millimeter range. This resolution limitation is determined by the currently available nanoparticles and the magnetic field gradient that can be technically realized. The data with high sensitivity exclusively with respect to the applied nanoparticles moreover allows statements about the quantitative distribution of the nanoparticles, which includes, however, only limited morphological information. For this reason, it is extremely difficult to unambiguously allocate the measured particle distribution to its morphological location of generation.
Other volumetric imaging methods, such as e.g. the MRI method that has been clinically used for a long time, are ideally suited for detecting high-resolution morphological information. MRI technology is based on a strong homogeneous magnetic field, the so-called polarization field (PF), and also on alternating electromagnetic fields in the radio frequency range which are used for resonantly exciting specific nuclei of the object under investigation (cf. reference [6]). The excited nuclei emit, in turn, alternating electromagnetic fields that induce electric signals in the receiver coil. When several magnetic field gradients are used, the signal is spatially encoded and can be reconstructed by means of suitable algorithms. MRI not only allows acquisition of anatomical information with high spatial resolution with diverse soft tissue contrasts but also offers further differentiated techniques that permit access to many physiological parameters such as e.g. water diffusion or permeability [6]. In MR spectroscopic imaging, it is additionally possible to spatially present metabolic and biochemical processes. In contrast to MPI, the MRI technique is a relatively insensitive and slow imaging method with acquisition times in the range of seconds to minutes.
Due to the unique properties of both volumetric imaging modalities, MPI and MRI are largely complementary with respect to their information content. A combination of both methods and synergetic utilization of their properties, the high sensitivity and also temporal resolution of the MPI technology and the diverse soft tissue contrasts and thus excellent morphological information of MRI technology, enables superior diagnostic informative value. A superposition/fusion of both complementary image data sets has been realized up to now only by means of two separate and independent modalities of MPI and MRI (cf. reference [3]) due to the global lack of availability of an integral device (hybrid device) combining these two modalities at the present time.
However, the use of two separate modalities involves some difficulties, i.a. the co-registration of both data sets with different reference coordinates. The co-registration is aggravated by shifting and deformation, which generally cannot be avoided due to relocation or transport of the object under investigation from the one modality to the other. The intermodality transport moreover prevents direct correlation of the two data sets in time. There are further logistical problems e.g. in case of studies on small animals, which require continuous anaesthetization of the test animal. The provision of two stand-alone modalities also results in high cost and extensive space requirements.
These difficulties were partially addressed in the earlier German patent application DE 10 2012 216 357.3, which was not published before the filing date of the present application. It describes an integrally structured hybrid system, in which the main magnet coil system has at least one element that generates a magnetic field and also generates a magnetic field portion both in the MRI volume under investigation and also in the MPI volume under investigation that is indispensable for both volumes under investigation. Such an integrally structured hybrid system generates a magnetic field behavior that suits both the requirements for MRI and also MPI modalities, wherein the spatial points of the centers of the two volumes under investigation do not coincide and the two volumes under investigation are not superimposed on each other. Due to these features, relocation of the measurement object is absolutely necessary, which aggravates co-registration of both data sets. Relocation also limits the chronological succession of both investigations.
US 2012/0119739 A1 also addresses a combination of MPI and MRI and the problems arising thereby, in particular, that the two measurement methods require coils for generating the magnetic field, which have clearly different geometries. These difficulties shall be solved by using pre-polarized MRI in accordance with the teaching of US 2012/0119739 A1.
In contrast thereto, it is the underlying purpose of the present invention to improve a device of the type described above in an inexpensive fashion with as simple technical means as possible in such a fashion that the above-described difficulties of a switchable combination of both modalities in an integrally structured hybrid device can be reduced or eliminated, thereby simplifying or even eliminating the need for relocation of the measurement object.